1. Field of the Invention
The invention relates to a leakage current suppressing circuit and a semiconductor chip, more particularly to a leakage current suppressing circuit and a semiconductor chip suppressing a leakage current via a pull-up resistor.
2. Description of the Related Art
A high definition multimedia interface (HDMI) is an audio/video communication interface for transmitting non-compressed encrypted digital data streams, and enables coupling of multiple audio/video sources (e.g., set top box, blue-ray disc player, etc.) to an audio device and/or a video monitor (e.g., digital television).
FIG. 1 is a schematic diagram showing an audio-video system composed of a plurality of audio-video devices, each of which includes an HDMI. With reference to FIG. 1, a consumer electronics control (CEC) signal line 11 is used for controlling all devices that are coupled to the HDMI, as set forth in the HDMI specification. In the example shown in FIG. 1, the audio-video devices include a camcorder 100, a digital video recorder (DVR) 200, a game console 300, and a television 400.
FIG. 2 is a circuit block diagram of two devices 12 and 13 connected by a CEC signal line 11. The devices 12, 13 may be any of the audio-video devices 100-400 of FIG. 1. In general, each of the devices 12, 13 includes a circuit board 121, 131, and a chip 122, 132 disposed on the circuit board 121, 131. Each chip 122, 132 uses an open drain or similar means to drive the CEC signal line 11 at an output terminal 123, 133 thereof. Therefore, the HDMI specification suggests adding a 27KΩ pull-up resistor 124, 134 on the circuit board 121, 131 for providing a high voltage level at the output terminal 123, 133. Resistance of each pull-up resistor 124, 134 has a drift that falls within the range of ±5%. When one of the devices 12, 13 (e.g., device 12) is turned off, while the other one of the devices 12, 13 (e.g., device 13) is turned on, it is necessary to avoid leakage of current from the turned-on device 13 into a power source 126 of the turned-off device 12 via the CEC signal line 11 and the pull-up resistor 124 of the turned-off device 12. To this end, the HDMI specification suggests adding a diode 125, 135 in series with the pull-up resistor 124, 134 on the circuit board 121, 131 so as to limit direction of current flow.
If the suggestions on the HDMI specification are adopted, manufacturing costs of the devices 12, 13 are increased due to the addition of the resistors 124, 125 and the relatively costly diodes 125, 135. Further, since voltage drop across each diode 125, 135 during forward bias is non-zero, equivalent resistance of the series-connected resistor 124, 134 and diode 125, 135 is greater than the actual resistance of the resistor 124, 134. In addition, the difference between the equivalent resistance and the actual resistance increases as voltage level (VDD) of the corresponding power source 126, 136 decreases.